1. Field of the Invention
This invention relates generally to a process for preparing aqueous compositions including titanium dioxide pigment, and more particularly to the dispersion and distribution of titanium dioxide particles in products formed by the loss of water from such aqueous compositions, including aqueous coating compositions.
2. Background of the Invention
Titanium dioxide has been for many years the pigment of choice for conferring opacity to plastic sheets and films, and particularly to coatings formed from coating compositions and paints. In paints titanium dioxide is typically the most expensive component of the formulation on a volume basis. It has been an ongoing objective of the coatings industry to achieve the desired degree of opacity in a coating while at the same time using as little titanium dioxide pigment as possible. One way in which this is done is by employing titanium dioxide which has an optimal average particle size and particle size distribution for scattering light. Another way of making efficient use of the titanium dioxide employed is by dispersing this pigment as well as possible.
Agglomerated or aggregated titanium dioxide pigment particles make less than their full potential contribution to the performance of the coatings, such as, for example, with regard to coating opacity and colorant potential. Such aggregates can also impair certain other properties of the coating or film such as, for example, strength and resistance properties. Titanium dioxide is often sold as a dry powder. As a practical matter, this powder must to be milled in a liquid medium to break up agglomerates and to obtain a colloidally stable dispersion.
In order to stabilize the coating formulation against settling or flocculation, a variety of surface active dispersing additives have been used. Coatings manufacturers have often used titanium dioxide as a dry powder, which is used directly in preparing coating compositions. In this case, a dispersing additive is usually added directly to the pigment and a small amount of water in a preliminary pigment "grind" step in which loose agglomerates of the pigment particles are broken up by mechanical shear forces. The dispersing additive typically remains in the mix through the subsequent steps in the coating preparation process and typically will be found in the fully formulated coating composition.
There has been an increasing tendency for titanium dioxide to be commercially supplied in concentrated slurry form, such as, for example, in water. However, since the titanium dioxide particles in the slurries are prone to aggregation upon storage, the slurries often need to be redispersed for maximum effectiveness before use in formulating a coating composition. Either the manufacturer of the titanium dioxide slurry or the end user of the slurry, and sometimes both, may perform the redispersion step. This step is another source of dispersing agent in the fully formulated coating.
The opacifying capability or hiding power of a paint or coating is a function of the spacing of the titanium pigment particles in the dried coating. The light scattering characteristics of titanium dioxide particles are well known. The average size and size distribution of titanium dioxide particles used for opacifying purposes has been highly optimized by the titanium dioxide manufacturers for maximum scattering. Maximum light scattering occurs when the titanium dioxide pigment particles have a diameter of from about 200 to about 250 nanometers and are spaced far apart from each other, on the order of a few particle diameters, so that there is minimal interference between the light scattering of neighboring particles.
In practice, however, for example in the formulation of paints, it is widely recognized that when enough titanium dioxide particles have been dispersed into the polymeric coating vehicle to yield films of acceptable opacity, the level of opacity which is achieved is significantly less than would be theoretically predicted from the light scattering potential of a single titanium dioxide particle multiplied by the total number of particles in the path of light passing through the film.
A number of factors have been identified which partially account for the diminution of opacity from the theoretical predictions. If two or more titanium dioxide particles are in actual contact with each other, or even if they are closer together than the optimum spacing distance, the particles will behave like a single aggregated larger particle and result in reduced light scattering. This occurs if the titanium dioxide particles are not adequately deaggregated during the dispersion process. However, even if the titanium dioxide particles are fully deaggregated in the dispersion process, a random distribution of particles will not provide the maximum scattering achievable in theory if the particles were optimally distributed.
In a related phenomenon, referred to as "crowding", titanium dioxide pigment particles are forced to be nearer to each other than the desired optimum merely by the lack of available space. This lack of available space may be caused by the space taken up by the other coating fillers and extenders which are of a comparable size to, or which are larger than, the pigment particles. In the case of polymeric binders, which are particulate in nature prior to film formation, such as, for example, latex or emulsion polymers, and in the case of nonaqueous dispersion polymers, the binder polymer particles themselves can crowd pigment particles, especially if they are of comparable or larger in size than the titanium dioxide particles.
The traditional guiding rule or goal in the formulation of practical, dispersed titanium dioxide-containing coatings, such as, for example, pigmented latex paints, is to make the titanium dioxide dispersion and the polymeric latex binder dispersions as colloidally stable and compatible with each other as possible, in the sense that they can be mixed without formation of coagulum or like aggregate or excessive increase in viscosity. It has heretofore been found that titanium dioxide particles cannot be effectively dispersed into aqueous latex compositions, by simply blending or directly mixing them into the aqueous polymeric latex composition. When such direct blending of titanium dioxide particles has been attempted, agglomeration of the titanium dioxide particles has resulted in the formation of grit or coagulum in the coating composition. Coatings containing grit or coagulum do not possess the desired degree of hiding or opacity. These coatings may also suffer the loss of other properties such as gloss, mechanical strength and impermeability. Even in the case of nonaqueous (solvent-borne) or 100% solids polymer compositions, high energy grinding or milling input has been found to be necessary to deagglomerate and wet out the titanium dioxide particles. Even with the use of such operations, inferior dispersions containing aggregates of titanium dioxide particles results unless significant quantities of dispersing, wetting or "coupling" agents are employed. However, even when such agents are used, the distribution of the titanium dioxide particles in the polymer system at best approaches that of a random distribution. As a result there exist a substantial number of titanium dioxide particles in close proximity to each other, and possibly in direct physical contact with each other.
In the case of latex paints and coatings, it is conventional practice to first form a stable aqueous dispersion of titanium dioxide pigment with other fillers or extenders. This dispersion, also known as a "mill base" or "grind," may contain water-miscible solvents, such as for example glycols and glycol ethers, and relatively low molecular weight water soluble polyelectrolytes as titanium dioxide pigment grinding aids or dispersants. Generally, these pigment dispersants are anionic polyelectrolytes. Many different types of such dispersants are known. For example, U.S. Pat. No. 2,930,775 discloses the water soluble salts of diisobutylene maleic anhydride copolymers having molecular weights between about 750 and 5,000 as dispersants when employed at concentrations of from about 0.05 to 4% on pigment weight. U.S. Pat. No. 4,102,843 and U.S. Reissue Pat. No. 31,936 disclose the use of water soluble salts of copolymers of hydroxyalkyl-(meth)acrylates and (meth)acrylic acid of molecular weights of from 500 to 15,000 at concentrations of from about 0.01 to 5% on pigment to produce glossy emulsion paints. U.S. Pat. No. 4,243,430 discloses a water-soluble dispersant comprising an addition copolymer comprising greater than 30% alpha, beta-unsaturated monovinylidene carboxylic acid, the copolymer having an apparent pK.sub.a between 6.0 and 7.5 and molecular weight between about 500 and 15,000, and forming a water soluble salt with zinc ammonia complex ion. Low molecular weight polyphosphate salts, such as potassium tripolyphosphate, are also used because they are relatively inexpensive, but they tend to have marginal hydrolytic stability.
The use of these and other polyelectrolyte dispersants is described in T. C. Patton, Paint Flow and Pigment Dispersion (Wiley Interscience, 2nd edition) 290-295 (1979). Also described therein (pages 468-497) are a number of milling devices used in the preparation of pigment dispersions. One such device which is commonly used in the manufacture of latex paints is the high speed disk disperser designed to develop high shearing forces in the pigment grinding step. Common practice is to use the device with dispersant to form a stable dispersion of titanium dioxide pigment, and then to add to the dispersion the aqueous latex polymer binder along with the other ingredients, such as for example thickeners and rheology modifiers, antifoaming agents, colorants, coalescing agents or temporary plasticizers for the latex polymer particles, and surfactants for substrate wetting and colorant compatability. The pigment dispersion process and the relation to flocculation and optical properties are discussed further in Treatise on Coatings, Vol. III, Part 1 (Marcel Decker) (1975); Pigment Handbook, Vol. I (2nd edition, Wiley Interscience) (1988), and Pigment Handbook, Vol. III (Wiley Interscience) (1973).
While these polymeric dispersants and procedures enable the formulation of practical aqueous formulations, they have certain undesirable characteristics. Because of their ionic nature, polyelectrolyte dispersants tend to impart water sensitivity to films, which can result in reduced resistance of the films to scrubbing, and increased swelling with a tendency towards delamination and blistering. Moreover, such polyelectrolyte stabilized dispersions of titanium dioxide particles are prone to flocculation and reaggregation by other ionic species in the aqueous formulation, such as, for example, from initiator residues and from soluble inorganic pigments, especially those which contribute multivalent ions such as zinc oxide and calcium carbonate. Furthermore, since films dry by water evaporation, the concentration of such ionic species in the coating or film increases upon drying, and an otherwise suitable dispersion may become aggregated during the drying process itself. The stability and instability of titanium dioxide dispersions in the presence of polyelectrolytes and multivalent metal ions has been described by Burlamacchi, et al., Colloids and Surfaces 7, 165 (1982).
Even in the unlikely case that a polyelectrolyte dispersant were to confer effective dispersion of titanium dioxide pigment particles to provide a dispersion of singlet particles, the range of distance of the repulsive forces between particles is very small, typically on the order of less than about 100 Angstroms (10 nanometers), in relation to titanium particle size (on the order of 250 nanometers). Consequently, the repulsive forces between particles in such dispersions would be incapable of maintaining any significant degree of spacing between the titanium dioxide particles to improve the scattering or hiding power of the dispersed titanium dioxide pigment, or to have a significant impact on the mechanical properties of the dried film.
A number of techniques have been proposed to disperse inorganic particles such as titanium dioxide particles in aqueous polymer containing coating compositions.
For example, U.S. Pat. No. 4,608,401 discloses a method for encapsulating solid particles by admixing the particles in an aqueous reaction medium with a water-insoluble polymerizable monomer in the presence of nonionic surface active stabilizing agent (such as a polyethoxylated alkylphenol containing at least about 8 carbon atoms in the alkyl group and preferably at least 40-50 ethylene oxide groups per molecule) to form a water-insoluble polymer free of ionic charge. A redox polymerization is employed which is free of ionic groups and does not release ionic groups in the reaction medium. The starting particles must be free of significant levels of ionic charge, either anionic or cationic, existing either from their structure or generated during their preparation and handling through electrolyte additions. The '401 patent states that "[p]articles which are charged have been found not to participate in the present encapsulation mechanism but, in fact, to severely inhibit the same, resulting in virtually immediate flocculation of the entire solids" (column 7, line 62--column 8, line 4).
Naturally agglomerated particulate materials are taught as being effectively dispersed in situ during the polymerization, eliminating the necessity for preliminary grinding and/or dispersion treatments. The '401 patent teaches that agglomerates of the inorganic pigment particles, which are present before and during the initiation of the polymerization reaction, are broken down or "exploded" apart during the polymerization. The behavior of the system has not been fully explained.
Latex paints prepared using pigment dispersed according to the procedure of the '401 patent are claimed to exhibit improved gloss, opacity, and scrub resistance relative to paints prepared by conventional means, although details for the preparation of the latter including the method of pigment dispersion and the characteristics of the latex polymer to be used, are not provided. The entire amount of polymer which appears in the final paints of the '401 patent is introduced in the polymerization process in the presence of the pigment. The process for achieving pigment encapsulation requires the handling of reactive monomers and other ancillary chemicals and the conducting of chemical reactions in the presence of pigment as an integral part of the process.
U.S. Pat. No. 4,025,483 is directed towards stabilizing aqueous titanium dioxide pigment slurries by blending plastic pigment with the slurry. At least about 10% of nonfilm-forming polymer latexes of 1,000-10,000 Angstroms in size, and having a glass transition temperature greater than 30.degree. C., are blended with about 10-50% water. The '483 patent discloses stabilizing such slurries against gravitational separation and sedimentation upon storage of the slurry. In the illustrative examples, Tamol.RTM. 731 surfactant is used at a concentration of 0.35% on pigment. The 1483 patent does not disclose or teach adsorption of the latex on the titanium dioxide particles. While the slurry containing latex in patent Example 1 has a total volume solids of about 41% compared to the control dispersion of Example 3 which has a total volume solids of about 31%, it is possible that increased volume solids alone might be expected to retard the rate of settling. While the systems which are compared are of approximately the same initial viscosity, the control system contains much more methyl cellulose thickener.
U.S. Pat. No. 4,315,959 relates to a process for coating microscopic substrates, such as pigment particles, dispersed in an aqueous medium containing a complex of a polyhydroxylated polymer, preferably polyvinyl alcohol ("PVA"), and a transition metal, most preferably copper II. The complex forms a layer on the particles which initiates polymerization of monomers such as acrylates and methacrylates. Example 1 discloses polymethyl methacrylate coated on titanium dioxide on a first layer of the PVA/Cu II complex. Tests such as electron microscopy and Hegman fineness show that the size of the treated particles was very nearly identical to the size of the untreated particles. Example 6 is directed to kaolin and talc coated with polymethyl methacrylate and show a higher modulus of elasticity relative to untreated materials in polymethylmethacrylate or using a known anchoring agent. As in the '401 patent, the process of achieving layering of a polymer on pigment involves conducting chemical reactions in the presence of the pigment. See also P. Goddard et al., Double Liaison, No. 387-388, p. II (1988).
U.S. Pat. No. 4,800,103 describes a process which involves treating a particulate inorganic material with the latex by mixing the latex with an aqueous suspension containing from 5-50% by weight of the inorganic material, adding a water-soluble salt having a multivalent cation, and adjusting the pH of the suspension to more than 4.0 in order to coagulate the latex-treated material to form an open three-dimensional network of larger flocs. The flocculated material is then dewatered and the resultant cake is dried and then pulverized. Alternatively, the latex may be added to a suspension of the inorganic material with a suitable dispersing agent for the inorganic material. The resultant mixture is then spray dried. An object of this invention is to prepare anhydrous compositions that may be easily incorporated in resins for castings, such as, for example, polyurethanes, unsaturated polyesters, acrylics, nylon and polypropylene.
U.S. Pat. No. 4,421,660 discloses the steps of emulsifying a hydrophobic, emulsion polymerizable monomer in an aqueous colloidal dispersion of discrete particles of an inorganic solid, and subjecting the resulting emulsion to emulsion polymerization conditions to form a stable, fluid, aqueous colloidal dispersion of the inorganic solid particles dispersed in a matrix of water-insoluble polymer of the hydrophobic monomer. An aqueous dispersion of the inorganic material is first made using surfactants such as fatty acid salts, polysoaps such as sodium polyacrylate, and especially polysoaps such as potassium salts of functionalized oligomers, such as for example the Uniroyal Chemical Polywet.TM. varieties. In Example 6 titanium dioxide of 200-400 nanometers in diameter is polymerized with a styrene/butyl acrylate composition to yield a bimodal dispersion of mean diameters of 650 nanometers and 110 nanometers. A coating, the composition of which is not described, prepared from the latex is reported as having excellent opacity. As in the prior references, the coating of the inorganic particles is achieved by an emulsion polymerization chemical reaction in the presence of the particles.
The attractive and repulsive forces which control the ability of particles of one type and size to adhere onto the surface of other particles have been the subject of extensive theoretical work and investigation with model systems, as evidenced by numerous publications. These publications typically refer to the phenomenon of particle-particle adhesion as "heterocoagulation", and discuss maximizing the freedom of dissimilar particles from undesirable heterocoagulation in very dilute systems. The theory of so called "heterocoagulation" processes has been described by an extension of the well-known colloidal stability theory of Derjaguin, Landau, verwey and Overbeek ("DLVO theory"). This extension is given by the Hogg, Healy and Furstenau equation who extended the DVLO theory to include the interaction between spherical colloidal particles of different radii, unequal surface potentials and differing London-van der Waals (i.e. Hamaker) constants, and is described in R. Buscall, et al., Polymer Colloids (Elsevier Applied Science Publications 1985) pages 89-90 and 165-167.
DLVO theory mathematically expresses a balance between attractive forces attributed to van der Waals forces and repulsive forces attributed to like electrical charges on the surfaces of interacting particles. Other types of interaction forces, for example steric repulsion and attraction due to dissolved polymer, can be incorporated into the basic theory at least semi-quantitatively. Investigators have shown the applicability of the theory in very dilute systems. Their conclusions are typically expressed in terms of particle collisions and minimum repulsive energy barriers between particles sufficient to overcome the attractive forces. Below this energy barrier there are too many collisions of particles with energies exceeding this minimum repulsive energy barrier to prevent coagulation. The relationship between this energy barrier and coagulation is taught as depending on several particle and medium variables which include medium dielectric constant, medium ionic strength, particle size, particle surface charge which may be expressed in terms of zeta potential, and the material attraction or Hamaker constant for colloidal materials immersed in the particular medium. No single parameter is, therefore, by itself a predictor of coagulation or stability.
If colloidal dispersions of particles differing in sign of charge are mixed, the usual result is a gross flocculation or coagulation. This result may be a desirable consequence in some circumstances, such as, for example, in instances where it is desired to purify water containing suspended matter or to isolate a bulk solid material from its colloidal suspension. If, however, the dispersions of particles having different sign of charge are mixed together under conditions of low particle concentration, and where one of the particle types is smaller than the other and present in greater number, then gross coagulation may be avoided and the smaller particles may form a monolayer on the larger ones.
However, the particle concentrations required to cleanly effect such a process, without forming significant quantities of coagulum or grit, are so low as to render such a process commercially impractical, since large volumes of liquid dispersions would have to be handled.
The DLVO theory and extensions thereof have been useful as a guide for interpreting and correlating aspects relating to the stability of small particles and dilute colloidal dispersions. These theories has been useful despite their quantitative limitations, and the fact that all of the necessary parameters for implementation, such as, for example, the material attraction or Hamaker constants, are not always known, or are not known with sufficient accuracy for all the materials of possible interest. The primary deficiency of the DLVO theory is that it is limited to the interactions of two isolated particles of the same type with each other in very dilute dispersions.
Despite extensive research relating to the theory of particle interaction and extensive work with model systems at low particle concentrations, as reflected by numerous publications in the field of colloidal stability and heterocoagulation, the utility of the DVLO and other theories as relating to the preparation of commercial dispersions containing high concentrations of inorganic particles has not been established.
There is a continuing need to improve the effective utilization of titanium dioxide in aqueous coating compositions and thereby to improve the opacity and other performance properties of coating compositions. In addition, there is a need for a method of minimizing the viscosity of titanium dioxide slurries, and to quickly prepare such slurries. Further, there is a need for a process to disperse titanium dioxide particles at high concentrations in coating compositions with a minimal amount of dispersing surfactant and with the substantial absence of grit. In particular, there is a need for a process for preparing stable, high solids, dispersions of microcomposite particles having polymer particles adsorbed onto a titanium dioxide particle.